Fever in a Baby Under 3 Months Old
The short answer
A fever in a baby under 3 months old (temperature of 100.4 degrees F / 38 degrees C or higher taken rectally) is always a medical emergency. Go to the emergency room immediately, regardless of how well your baby appears. Young babies' immune systems cannot fight infections as effectively as older children, and a fever could indicate a serious bacterial infection (urinary tract infection, bacteremia, or meningitis) that needs urgent treatment. Do NOT wait to see if the fever goes down. Do NOT give fever medication and stay home - go to the ER first.
Parents everywhere have the same worry. You are doing the right thing by looking into it.
By Age
What to expect by age
0-12 months
For babies under 28 days old, a fever is treated as a potential medical emergency requiring hospitalization for blood tests, urine tests, and often a lumbar puncture (spinal tap) to rule out meningitis. Antibiotics are typically started immediately while awaiting test results. For babies 29 days to 3 months, fever evaluation typically includes blood tests and urine tests, with the decision about hospitalization and lumbar puncture based on the results and how the baby looks. After 3 months, fever is still taken seriously but is less likely to indicate a severe bacterial infection. Always take a rectal temperature for the most accurate reading in young babies - ear, forehead, and axillary (armpit) temperatures are less reliable.
What Should You Do?
When to take action
- There is no "normal" fever in a baby under 3 months - all fevers in this age group require immediate medical evaluation
- A fever in a baby under 3 months always requires emergency evaluation, not a routine office call
- ANY temperature of 100.4 degrees F (38 degrees C) or higher (taken rectally) in a baby under 3 months - go to the ER immediately
- A temperature below 97 degrees F (36.1 degrees C) in a newborn can also be concerning and should be evaluated
- Your newborn is feeding poorly, unusually sleepy, irritable, or "just not right" even without a confirmed fever
- Your baby under 3 months has been exposed to someone with a known infection and develops any symptoms
Sources
Related Resources
Trust your instincts. If something feels wrong, reach out to your pediatrician.
Worrying about your baby means you care. That is a good thing.
Related Medical Concerns
Baby Has a High Fever Over 104 Degrees F
A fever above 104 degrees F (40 degrees C) in a baby or toddler can be alarming, but the height of the fever alone does not necessarily indicate a more serious illness. Many common childhood viral infections (like roseola) can cause high fevers. What matters more than the number is how your child looks and behaves. A child with a 104 degree F fever who is still alert, making eye contact, and drinking fluids is less concerning than a child with a 102 degree F fever who is limp and unresponsive. However, fevers above 104 degrees F should always be discussed with your pediatrician.
Ibuprofen and Acetaminophen Dosing Safety
Acetaminophen (Tylenol) can be given to babies 2 months and older. Ibuprofen (Motrin/Advil) should NOT be given to babies under 6 months. Dosing is based on your child's WEIGHT, not age - always use the dosing syringe that comes with the product and follow the weight-based chart on the packaging. Never give aspirin to children under 18 (risk of Reye syndrome). When in doubt about dosing, call your pediatrician. Alternating acetaminophen and ibuprofen can be effective for fever but increases the risk of dosing errors - only do this under your pediatrician's guidance.
Cold vs. Flu in Baby - How to Tell the Difference
Colds and flu are both respiratory illnesses but are caused by different viruses and differ in severity. A cold comes on gradually with runny nose, sneezing, and mild cough. The flu hits suddenly with high fever, body aches, extreme fatigue, and a dry cough. Babies and children under 5 are at higher risk for flu complications. The flu can be treated with antiviral medication (like oseltamivir/Tamiflu) if started within 48 hours of symptom onset, which is why early evaluation is important. The best prevention is the annual flu vaccine, recommended for all children 6 months and older.
My Baby's Head Shape Looks Abnormal
Many babies develop temporary head shape irregularities that are completely normal. A cone-shaped head from vaginal delivery reshapes within days. Mild positional flattening (plagiocephaly) from sleeping on the back is very common and usually improves with repositioning and tummy time. However, head shape changes involving ridges, a persistently bulging fontanelle, or rapid head growth changes should be evaluated to rule out craniosynostosis.
Achondroplasia (Dwarfism) in Babies
Achondroplasia is the most common form of short-limbed dwarfism, affecting about 1 in 15,000 to 40,000 births. It is caused by a mutation in the FGFR3 gene and is usually apparent at birth with characteristic features including short limbs, a larger head, and a prominent forehead. Intelligence is normal. With monitoring for specific complications and supportive care, children with achondroplasia lead full, active, and independent lives.
Adenoid Hypertrophy and Breathing
Adenoids are lymphoid tissue located behind the nose that help fight infection in young children. When adenoids become enlarged (adenoid hypertrophy), they can block the nasal airway, causing chronic mouth breathing, snoring, nasal speech, and sleep-disordered breathing. Enlarged adenoids are most common between ages 2-7 and are a leading cause of obstructive sleep apnea in young children. Treatment ranges from watchful waiting and nasal steroids to surgical removal (adenoidectomy) if breathing or sleep is significantly affected.